1. Field
The invention relates to compositions and methods for manufacturing zirconium-based fuel elements for nuclear water reactors and, more particularly, to zirconium alloy claddings including a chromium coating and an intermediate metallic layer deposited between the cladding and the chromium coating to enhance corrosion resistance under nuclear reactor accident conditions and during normal operation.
2. Description of Related Art
In a typical commercial nuclear water reactor, such as a pressurized water reactor (PWR), heavy water reactor (e.g., a CANDU) or a boiling water reactor (BWR), the reactor core includes a large number of fuel assemblies, each of which is composed of a plurality, e.g., bundles or assemblies, of elongated fuel elements or fuel rods. Fuel assemblies vary in size and design depending on the desired size of the reactor and the core.
The fuel rods each contain nuclear fuel fissile material, such as at least one of uranium dioxide (UO2), plutonium dioxide (PuO2), thorium dioxide (ThO2), uranium nitride (UN) and uranium silicide (U3Si2) and mixtures thereof. At least a portion of the fuel rods can also include neutron absorbing material, such as, boron or boron compounds, gadolinium or gadolinium compounds, erbium or erbium compounds and the like. The neutron absorbing material may be present on or in pellets in the form of a stack of nuclear fuel pellets. Annular or particle forms of fuel also can be used.
The fuel is encased in sealed tubes, commonly referred to as the fuel cladding. Each of the fuel rods has a cladding that acts as containment to hold the fissile material. The fuel rods are grouped together in an array which is organized to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus, the release of a large amount of energy in the form of heat. The cladding maintains the fuel in a position, for which controlled fission can proceed and generate heat. A coolant, such as water, is pumped through the reactor core to extract the heat generated in the reactor core for the production of useful work such as electricity. The cladding then transfers the heat from the fuel to pressurized water that circulates around the primary loop of the reactor coolant system. The heated water in the primary loop is used to boil water in a steam generator and the steam is then expanded in a turbine that powers an electrical generator. Alternatively, the water circulating through the reactor may be allowed to boil to generate steam directly, which is then expanded in a turbine.
In a typical commercial nuclear reactor, the fuel assemblies in the core each have top and bottom nozzles. A plurality of elongated transversely spaced guide thimbles extends longitudinally between the nozzles. The plurality of elongated fuel elements or rods which compose the fuel assemblies are transversely spaced apart from one another and from the guide thimbles. A plurality of transverse support grids are axially spaced along and attached to the guide thimbles. The grids are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods, and induce mixing of the coolant.
FIG. 1 shows an exemplary reactor pressure vessel 10 and nuclear core 14. The nuclear core 14 includes a plurality of parallel, vertical, co-extending fuel assemblies 22. For purpose of this description, the other vessel internal structures can be divided into lower internals 24 and upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 1), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 1, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the vessel and the core barrel 32, is turned 180° in a lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, a lower core support plate having the same elevation as 37. The coolant flow through the core and surrounding area 38 is typically large, on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially outward to one or more outlet nozzles 44.
One of the exemplary fuel assemblies 22 as shown in FIG. 1 is shown in more detail in FIG. 2. Each of the fuel assemblies 22 includes radially-extending flukes or arms 52 and fuel rods 66 grouped in an array thereof. The fuel rods 66 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. At its lower end, a bottom nozzle 58 supports each of the fuel assemblies 22 on a lower core plate 36. At its upper end, each of the fuel assemblies 22 includes a top nozzle 62. An instrumentation tube 68 is located in the center and extends between and is mounted to the bottom and top nozzles 58 and 62. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74, respectively. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the reactor.
Each of the fuel rods 66 includes a cladding which surrounds the pellets to function as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system. The cladding on the fuel rods may be composed of a zirconium (Zr) based alloy. The cladding may include Zr and as much as about two percent by weight of other metals, such as niobium (Nb), tin (Sn), iron (Fe), chromium (Cr) and combinations thereof.
It is known in the art that there are various concerns relating to nuclear fuel rod cladding, including rapid corrosion of the Zr alloy tube at elevated temperatures associated with an accident scenario. In the event of an accident such as a Loss of Coolant Accident, temperatures inside the reactor core can exceed 1200° C. At very high temperatures, Zr rapidly oxidizes in the presence of steam which causes degradation of the fuel rods and production of large amounts of hydrogen which can lead to chemical explosions. Furthermore, breakdown of the fuel/cladding barrier in combination with explosions can cause wide-spread contamination of the plant and surrounding environment.
Applying to the outside surface of the fuel element an oxidation resistant coating that is capable of withstanding high temperatures, e.g., about 1200° C. and above, can provide operators and safety systems a longer time period to restore the reactor core to safe conditions and therefore, at least reduce and potentially avoid the potential negative consequences associated with Zr oxidation and fuel rod degradation in an accident scenario.
It is an object of this invention to provide compositions and methods for manufacturing a Zr alloy nuclear fuel cladding having an intermediate metallic layer between the Zr alloy and a chromium coating which is applied to the Zr alloy nuclear fuel element. The intermediate metallic layer and the chromium coating on the Zr alloy substrate enhance corrosion resistance by imparting a protective chromium oxide layer after exposure to steam or water. Known conventional deposition apparatus and techniques are used to apply the adherent intermediate metallic layer and chromium coating.